Fifthly, That Iron is converted into Steel by means of the incorporation of
certain salts, with which it is kept a certain time in the fire.
Sixthly, That any Iron may, in a very little time, be _case hardned_, as
the Trades-men call it, by casing the iron to be hardned with clay, and
putting between the clay and iron a good quantity of a mixture of _Urine_,
_Soot_, _Sea-salt_, and _Horses hoofs_ (all which contein great quantities
of Saline bodies) and then putting the case into a good strong fire, and
keeping it in a considerable degree of heat for a good while, and
afterwards heating, and quenching or cooling it suddenly in cold water.
Seventhly, That all kind of vitrify'd substances, by being suddenly cool'd,
become very hard and brittle. And thence arises the pretty _Phaenomena_ of
the Glass Drops, which I have already further explained in its own place.
Eighthly, That those metals which are not so apt to vitrifie, do not
acquire any hardness by quenching in water, as Silver, Gold, &c.
These considerations premis'd, will, I suppose, make way for the more easie
reception of this following Explication of the _Phaenomena_ of hardned and
temper'd Steel. That Steel is a substance made out of Iron, by means of a
certain proportionate _Vitrification_ of several parts, which are so
curiously and proportionately mixt with the more tough and unalter'd parts
of the Iron, that when by the great heat of the fire this vitrify'd
substance is melted, and consequently rarify'd, and thereby the pores of
the Iron are more open, if then by means of dipping it in cold water it be
suddenly cold, and the parts hardned, that is, stay'd in that same degree
of _Expansion_ they were in when hot, the parts become very hard and
brittle, and that upon the same account almost as small parcels of glass
quenched in water grow brittle, which we have already explicated.
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